EoW March 2009

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EuroWire – March 2009 94
technical article
Calcium-zinc systems, as the recent
increase has shown, are a good replace-
ment for lead-based stabilisers. The main
application areas where Ca-Zn systems
have highest penetration are wire and
cable and automobile interiors, followed
by pipes and profiles.
The selection of metallic compounds as
non-lead stabilisers was based on the
fact that their effect on the human body
is slight, and that there was thus little
likelihood of their becoming subject to
regulation and limitation in the future.
Stabilisers made from these metals were
combined and a PVC resin with a non-lead
stabiliser was developed for use in wire
insulation and sheathing.
3.2 The function of flame-retardants
in PVC
The process of combustion can be
described in the following steps:
heating •
decomposition (pyrolysis)
•
ignition and combustion •
propagation, with thermal feedback •
The heating of the material by external
thermal sources increases the temperature
of the material, with a speed that depends
on the intensity of the heat emitted,
the thermal conductivity characteristic
of the material, the latent heats of
fusion and vaporisation and the heat of
decomposition. Once reaching a sufficient
temperature the material begins to
degrade, forming gaseous mixtures and
liquids. These mixtures are formed with a
speed that depends on the intensity with
which the polymer material is heated.
The concentration of the decomposition
products, blending with surrounding air,
increases until falling back in the inflam-
mability interval. The presence in this
situation of a source of heat makes the
ignition of the mixture. The produced heat
is in part irradiated to the material (thermal
feedback), so that it continues to pyrolysis.
The action of a flame retardant consists in
eliminating or limiting one of the factors,
acting in a physical or chemical way or
both, on the liquid, solid and gaseous
products originated in the process.
The physical action is of three types:
cooling the process of thermal feed- •
back, that it fails to supply the heat
necessary to progress the pyrolysis of
the polymer material
dilution of the combustion mixture •
formation of a protecting layer, where •
the solid polymer material is shielded
in oxygen from the rich gaseous
phase, by means of a solid or gaseous
protecting layer. It reduces heat to the
polymer, with a consequent slowing
down of pyrolysis and lessening
the contribution of oxygen to the
combustion process
The chemical action can be distinguished
in:
reaction in phase gas: the radicals •
generate from the flame retardant
chemically to act on the combustion
process
reaction in condensed phase can •
be carried out in two ways. The first
consists in forming a protecting
carbonic layer (char) on the surface of
the polymer, having the characteristics
of a thermal insulator and to act as
a barrier between the products of
pyrolysis and oxygen. The second is
that this layer increases and delays the
process of thermal feedback
Flame-retardants can be included in the
material in several ways:
reactive: react chemically with the
•
polymer
additive: blended with the polymer •
reactive and additive: present in the •
material in both ways
The choice of flame retardant is influenced
by:
toxicity •
biodegradability •
heat stability in the polymer •
Antimony Trioxide (Sb2O3) is normally
added in order to reduce the flammability
of plasticised PVC; however Sb 2O3 enhances
the stop of the radical chain mechanism in
the gas phase, and increases the amount
of smoke generated in case of fire.
Many PVC processors have expressed
interest in alternative flame-retardant
additives that provide a reduction in
flammability without themselves producing
toxic or corrosive components. The flame
retardant should not negatively influence
the specific characteristic of the PVC.
It is desirable that any improvement in
flame retardancy is combined with a
decrease in smoke density. In a fire event,
PVC releases Hydrogen Chloride (HCl),
with the humidity always present in the
air. Calcium carbonate is normally used in
PVC as an acid scavenger and a cost saving
filler. The ideal flame retardant should also
possess these benefits.
3.3 Study of possible incorporation of
nano-filler in PVC
Recently there has been much interest in
polymer nanocomposites (PNC), especially
polymer/clay nanocomposites. Three main
types of nanocomposites can be obtained
when a layered silicate is dispersed into a
polymer matrix.
This depends on the nature of the
components used including polymer
matrix, layered silicate and organic cation.
If the polymer cannot intercalate between
the silicate sheets, a microcomposite is
obtained. The phase-separated composite
that is obtained has the same properties
as traditional microcomposites. Beyond
this traditional class of polymer-filler
composites, two types of nanocomposites
can be obtained:
intercalated structures are formed •
when a single (or sometimes more)
extended polymer chain is intercalated
(sandwiched) between the silicate
layers. The result is a well-ordered
multilayer structure of alternating
polymeric and inorganic layers
exfoliated or de-laminated structures •
are obtained when the silicates are
completely and uniformly dispersed
in the continuous polymer matrix.
The de-lamination configuration is of
particular interest because it maximizes
the polymer-clay interactions, making
the entire surface of the layers available
for the polymer. This should lead to the
most significant changes in mechanical
and physical properties
In order to characterise the structures of
nanocomposites two complementary
analytical techniques are used. X-ray
diffraction (XRD) is used to identify
intercalated structures by determination
of the interlayer spacing. Nanocomposites
can demonstrate significant improvements,
compared to virgin polymers, with
the content of the modified layered
silicates in the 2-10 wt% range. There are
improvements in:
mechanical properties, such as tension •
compression, bending and fracture •
barrier properties, such as permeability •
and solvent resistance
optical properties •
ionic conductivity
•
Polymer combustion cycle diagram ▲ ▲
Volatiles
Oxygen
Flame Products
Heat
Polymer
Polymer
combustion
cycle,
Dispersion
Gas phase
Char
Intercalated
(nanocomposite)
Exfoliated
(nanocomposite)
Phase separated
(microcomposite)
Layered silicate Polymer
Diagram showing the three main types of ▲ ▲
nanocomposites which can be obtained when a
layered silicate is dispersed into a polymer matrix